1. Field of the Invention
The present invention relates to a three-phase brushless motor driving device which can immediately and stably start without a rotor position sensor and a motor driving method at the time of start.
2. Description of the Related Art
According to a brushless motor, in order to apply a stable torque to a rotor by selecting an appropriate winding of a stator to apply a current thereto, information of a relative position of electrical angle of the rotor with respect to the stator is needed. Various kinds of rotor position sensors to find the relative position of the electrical angle of the rotor with respect to the stator are used. Meanwhile, a sensorless driving technique which does not need the rotor position sensor has been developed in view of reliability or costs or environment resistance. According to such sensorless driving technique, it is well known that the rotor position is detected by reading a counter electromotive voltage generated in a stator phase winding while the rotor is rotated.
However, since the counter electromotive voltage is not generated when the rotor is stopped, the rotor position cannot be detected by the above method when the rotor is stopped. Thus, various kinds of methods of detecting the rotor position when the rotor is stopped have been proposed. For example, there is disclosed a method of detecting a rotor position from a stator phase which is sequentially selected and generates the highest amplitude of a current which flows when a rotor position search pulse is applied, as shown in Japanese Patent Publication No. 2547778. Furthermore, there is disclosed a method of finding a rotor position based on a result in which a polarity of a difference in response signal is sequentially detected by selecting a stator phase and applying a rotor position search pulse thereto in forward and backward directions as shown in Japanese Patent Publication No. 8-13196.
However, according to the method in the Japanese Patent Publication No. 2547778, there is a problem in accuracy in finding the peak value of the pulse current which flow when the rotor position search pulse is applied. Furthermore, as shown in FIG. 9, since a difference in pulse current peak value depending on the rotor position is small in the phase, it is necessary that variation in electric and magnetic characteristics that the stator and the rotor originally contain has to be small in the phase as a precondition. Therefore, the method is not suitable for an inexpensive motor in which characteristics with respect to each phase are not managed well. In addition, in case of a motor in which an inductance of a winding is low to attain high-speed characteristics, a pulse current is increased by itself, a current value to obtain the difference in pulse current peak value becomes high by itself. In addition, according to the method in the Japanese Patent Publication No. 8-13196, it is necessary to find the difference in response signal between positive and negative polarities of the rotor position search pulse signal, so that a table is needed to refer to a polarity combination of the response signal difference every phase. Thus, in order to implement the above method, an arithmetic processing ability is needed, which is not suitable for a case where controllability by a motor only is required or a case of an inexpensive motor driving system.